1. Field of the Invention
Example embodiments of the present invention relate to methods of forming a thin film. More particularly, example embodiments of the present invention relate to methods of forming a thin film having improved electrical characteristics, including a high dielectric constant through use of an atomic layer deposition (ALD) process, and to methods of manufacturing a gate structure, and a capacitor using the same.
2. Description of the Related Art
As semiconductor devices have become more highly integrated, the size of the cell area of these semiconductor devices has been significantly reduced. Thus, forming a capacitor in the cell area of a semiconductor device such as a dynamic random-access memory (DRAM) device, wherein the capacitors have the requisite capacitance for properly operating the semiconductor device has now become more difficult to achieve due to the minute size of the cell area of these semiconductor devices.
Generally, the capacitance (C) of a capacitor is in proportion to the dielectric constant (∈) and the area (A) of the dielectric layer, whereas the capacitance (C) of the capacitor is inversely proportional to the distance (d) between electrodes in accordance with the following equation (1):C=∈·A/d  [Equation 1]
When the dielectric layer of the capacitor is formed using silicon oxide or silicon nitride, the lower electrode of the capacitor generally has a cylindrical shape or a fin shape for increasing the capacitance (C) of the capacitor. However, the cylindrical or fin shape of the lower electrode is a relatively complicated shape, which in turn makes it more difficult to manufacture the lower electrodes precisely.
To solve the above-mentioned difficulty, a method of forming a dielectric layer using a high-k material instead of silicon oxide or silicon nitride has been researched. Examples of high-k materials include aluminum oxide (Al2O3), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), zirconium oxide (ZrO2), or titanium oxide (TiO2). The high-k material has a dielectric constant of about 10 to about 114, which is about 2.5 to about 30 times larger than the dielectric constant for silicon dioxide of 3.9.
A thin film such as a dielectric layer is usually formed by deposition processes such as a chemical-vapor deposition (CVD) process, a low-pressure chemical-vapor deposition (LPCVD) process, a plasma-enhanced chemical-vapor deposition (PECVD) process, or a sputtering process.
However, since the above deposition processes are generally performed at a relatively high temperature, thermal damage may occur to a thin film formed by any of these processes. Additionally, a thin film formed by a CVD process may also be formed having an irregular thickness, as well as poor step coverage.
In contrast, an atomic layer deposition (ALD) process is performed at a temperature that is relatively lower than the temperature used in a CVD process. Moreover, a thin film formed by an ALD process is formed having good step coverage. Thus, ALD processes have become the preferred new deposition process for forming a thin film.
For example, ALD processes have recently been used in forming a high-k material layer. The precursor used for forming the high-k material layer in an ALD process should, however, meet certain recommended conditions as set forth below.
First, the precursor should have a high-saturation vapor pressure at a low temperature and also be chemically and thermally stable. Moreover, ligands coordinated to a metal of the precursor should be promptly and clearly separated from the precursor. Additionally, organic ingredients of the precursor should not remain in the high-k material layer after a formation of the high-k material layer. The precursor should have a liquid phase at room temperature and also be nontoxic. Furthermore, the precursor should be deposited at a high rate.
It is noted however that conventional precursors such as alkyl metal, metal alkoxide, metal halide and β-diketonate may not sufficiently meet the above-mentioned recommended conditions for a precursor to be used in an ALD process for forming a thin film. For example, an alkyl metal, such as Pb(C2H5)4, is toxic and explosive. Further, since a metal alkoxide is sensitive to moisture, the metal in the metal alkoxide is prone to binding with a hydrogen or hydroxyl group, thereby leading to undesired impurities such as metal hydroxide being contained in a thin film. Moreover, a β-diketonate precursor has a relatively low vapor pressure and is in a solid phase at room temperature, thereby making β-diketonate an undesirable precursor for use in forming a thin film.
Meanwhile, fluorine β-diketonate, which is a more volatile type of precursor than those precursors already mentioned has been researched as well. Examples of fluorine β-diketonate precursors include hexafluoro pentanedionate (HFAC) and heptafluoro dimethyloctanedionate (HFOD). However, fluorine β-diketonate precursors have a poor reactivity relative to reactants so that ligands are not easily removed from metal in fluorine β-diketonate. In addition, fluorine β-diketonate precursors have a high molecular weight, resulting in a low deposition rate.
Accordingly, there is a need for a precursor for use in an ALD process for forming a thin film that provides improved electrical characteristics such as good step coverage, low leakage current, and a high dielectric constant to a semiconductor device. In particular, there is a need for a metal precursor for forming a thin film which may serve as a gate insulation layer of a gate structure or a dielectric layer of a capacitor and wherein the precursor preferably has the following characteristics of being thermally and chemically stable, not sensitive to moisture, the ability to exist in a liquid phase at room temperature, and has a high reactivity relative to reactants.